1. Field of the Invention
This invention relates generally to semiconductor diffraction gratings and, more particularly, to a semiconductor structure that includes semiconductor epitaxial layers lattice matched to each other and a diffraction grating therebetween, where the semiconductor layers and the diffraction grating material have a greater difference in their indexes of refraction than the difference in the indexes of refraction of the semiconductor layers.
2. Discussion of the Related Art
There is a need in the art for optical semiconductor diffraction gratings in certain optical semiconductor devices, such as distributed feedback (DFB) optical filters, optical couplers, etc. A conventional optical semiconductor diffraction grating will typically include a semiconductor waveguide layer positioned between outer cladding layers, where the waveguide layer has a higher index of refraction than the cladding layers so that light propagates down the waveguide layer by reflecting off of the cladding/waveguide interfaces and is trapped therein. The diffraction grating is formed at the interface between one of the cladding layers and the waveguide layer by fabricating a ripple or corrugated structure on one of either the waveguide surface or the cladding layer surface so that as the light is reflected off of the interface, it interacts with the grating. Thus, the diffraction layer is the periodic longitudinal index difference between the peaks and troughs defined between the semiconductor layers forming a grating region. As the light propagates down the waveguide layer, the wavelength of light related to the periodic index change or spatial period of the peaks in the diffraction layer is reflected backwards or transmitted through the waveguide layer in such a manner that it is separated from the other wavelengths of light to provide for example, optical filtering.
Processing techniques for fabricating the conventional grating interfaces in semiconductor devices are well established. The grating can be formed on the waveguide layer or cladding layer surface by direct electron beam writing or holography patterning, both well understood to those skilled in the art. In order to make the interfaces between the waveguide layer and the cladding layers of a high optical quality with minimum defects and imperfections, it is desirable to use a semiconductor epitaxial growth process, such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), to provide crystal nucleation and growth and lattice matching between these layers. Because the waveguide layer must have a higher index of refraction than the cladding layers, the waveguide layer must be made of a different semiconductor material or material composition than the cladding layers. However, to provide the necessary crystal structure and lattice matching during the semiconductor fabrication process, the waveguide layer and the cladding layers must be compatible to allow the crystal growth process to occur.
The interaction of the optical modes in the waveguide layer is a function of the differences in the indexes of refraction in the diffraction layer formed by the semiconductor layers, which is characterized by a grating coupling coefficient. The semiconductor materials that are compatible and satisfy the crystal growth requirement have nearly the same indexes of refraction, and thus, the optical wavelength separation capability provided by the diffraction grating layer in these devices is limited. In other words, because the crystal growth process requires that the waveguide and cladding materials have nearly the same index of refraction, the optical filtering capability, or other optical wavelength separation process, is limited. Typical semiconductor indexes of refraction are approximately 3, and the difference between compatible semiconductor materials is usually at most about 0.5.
The small difference in the indexes of refraction between the waveguide layer and the cladding layers is adequate for many applications, such as optical mode pumping in a laser, but for other applications, such as optical filtering, a larger difference between these indices of refraction is desirable. In many applications, a significant improvement in device performance would be achieved if it were possible to fabricate gratings with much larger coupling coefficients.
To provide semiconductor diffraction gratings of the type discussed above that have a much greater difference between the indexes of refraction within the grating layer, it has heretofore been known to use a wafer-to-wafer bonding technique to bond the waveguide layer and cladding layers together that eliminates some of the restrictions imposed on the semiconductor growth fabrication processes. Different wafer-to-wafer bonding techniques are known in the art, where separate semiconductor structures are adhered together in a non-crystal growth process. By bonding a semiconductor structure to another semiconductor structure that includes the diffraction ripples, an interface is created where air gaps are defined between the peaks in the ripple structure. Therefore, as the optical beam propagates down the waveguide and interacts with the diffraction grating layer, the optical beam sees alternating regions of air and semiconductor material. Because the index of refraction of air is one, there is a significant difference between the materials that define the grating, providing increased filtering capabilities.
Wafer-to-wafer bonding, however, has a number of drawbacks making this technique somewhat undesirable for fabricating optical diffraction gratings. Particularly, the wafer-to-wafer bonding process introduces strain between the crystalline structure of the two semiconductor layers that affects the optical interaction in the diffraction layer. Additionally, defects and impurities present at the interface can affect the optical integrity as a result of the bonding process that would not be present during a crystalline growth process. Additionally, the wafer-to-wafer bonding process is relatively expensive to implement, and thus adds a significant level of cost above the typical diffraction grating fabrication process.
What is needed is a process for making an optical diffraction grating that employs semiconductor crystal growth processes, and provides a relatively significant difference between the indexes of refraction between the waveguide layer and the cladding layers in a diffraction grating layer for increased optical filtering.